1. Field of the Invention
The loudspeaker system and method of the present invention relate to a sound system for generating diffuse background or masking noise in an office area or the like, generally for the purpose of covering or masking conversation in an open area and providing speech privacy to people sharing a large, open workspace. The loudspeaker system of the present invention is ideally suited for mounting in an overhead plenum, above a suspended tile ceiling or behind an architectural barrier separating a plenum from a workspace.
2. Discussion of the Prior Art
Architects and designers of large office spaces have largely abandoned the practice of placing each desk in its own small office. Instead, modern office arrangements usually include large, spacious, open floors shared by many desks, thereby (theoretically) providing enhanced efficiency and an informal atmosphere. One drawback of the new open plan office design is that privacy of conversation automatically provided by smaller individual offices is lost, since the conversation between workers or over the telephone is readily overheard and may provide a distracting intrusion not appreciated by adjacent workers. Distractions such as operation of business machines, telephones ringing and other extraneous noises may tend to lower productivity. The open plan concept has gone beyond the office and is finding acceptance in hospital patient rooms where, again, privacy is lost. In the hospital ward, each patient should be isolated from the sounds of other patients including conversations and TV sets. In schools, the problem of audible distractions is also difficult to address because one large room may be shared by several classes. Each classes' space must be acoustically contiguous (so that each student in a class can hear the teacher) but must also be acoustically separated from adjacent classes to minimize distractions.
The use of sound absorbing acoustical material is a basic element in the design of work spaces. Carpeting, wall and ceiling acoustical treatments are common; additionally, panels and sound barriers are casually arranged to aid in separation of space. These measures cannot provide an adequate solution, however, since they do not provide a sufficient amount of attenuation for all distracting noises.
Most open plan office spaces include a suspended ceiling where space above the ceiling is defined as a plenum in which office services are channeled. Sprinkler pipes, water pipes, air conditioning duct work, electrical conduits, telephone cables, computer network cables and many other mechanical and electrical services are routed through the plenum space.
It is well known to provide background noise generation systems for the purpose of masking conversations or other distracting noises. Several problems are confronted when attempting to design and install effective background noise masking systems, however.
The masking noise should be uniformly distributed throughout the space in order to achieve satisfactory masking results. Ideally, background masking noise is a broad spectrum, uniformly distributed, diffuse sound field of uniform intensity and is substantially imperceptible to those in the treated space. If the masking noise is not uniformly distributed or diffused throughout the work space, masking tends to be less effective in a first area and more effective in a second area; a person walking through a work space from the first area to the second area is subjected to different intensities of masking noise and thus is more likely to become conscious of and distracted by the masking noise. Because of this problem, masking systems employing loudspeakers radiating directly into the work space from the ceiling tend to be particularly ineffective and distracting.
There are prior art systems utilizing conventional sound system components (such as cone diaphragm loudspeakers) installed in the plenum spaces above the open plan office ceilings to position the speakers in an attempt to use plenum space as a mixing chamber for masking noise where, in theory, the masking noise from several loudspeakers mixes and then filters down uniformly through the ceiling and into the office space. Unfortunately, such installations tend to provide poor masking performance since the plenum is usually obstructed by duct work or the like and since the plenum may or may not be sufficiently acoustically reflective to provide adequate mixing. Insulated air conditioning ducts and other equipment in the plenum tends to interfere with distribution and mixing of the sound and provide poor mixing performance. By way of example, U.S. Pat. No. 3,985,957, to William R. Torn, discloses a structure including clusters of speakers mounted in the plenum above an office space. Each cluster has two cone diaphragm speakers in a prism-shaped cabinet symmetrically disposed about a vertical axis. The sound masking system of Torn requires that a plurality of clusters be employed to cover quiet regions which may develop below a cluster. Torn's sound masking system requires that the plenum region be relatively free of obstructing materials which would tend to interfere with the reflecting and mixing of masking sound before propagation down through the ceiling tiles into the office space. As shown in FIGS. 1a-1h included herewith, cone diaphragm loud speakers necessarily provide a substantially more directional output at higher frequencies (as compared to lower frequencies) thereby providing frequency dependent masking sound radiation (FIGS. 1a-1h are polar plots of sound pressure level as a function of angle at 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, 8000 Hz and 16000 Hz, respectively, for a conventional loudspeaker with a cone diaphragm). Since it is desired to provide a rather uniform pink or white noise for masking of conversation or the like, frequency dependant behavior may prove to be troublesome for an installer in trying to implement the sound masking system. Additionally, an installer working with the Torn system is required to suspend a plurality of loudspeaker cabinets having what may be very heavy loudspeaker drivers and a cabinet in the plenum space. As shown in Torn FIGS. 1, 2 and 3, the loudspeaker cabinets or clusters are preferably suspended from the ceiling above the plenum by chain, cable or the like and so must be held in place while being installed. Another problem associated with the Torn system is that most loudspeaker drivers are fabricated from pulp paper or plastic cone materials and so tend to be flammable. Most jurisdictions require that Underwriters Laboratories (UL) approved components be installed in a plenum, since any fire breaking out in the plenum space could travel quickly through a building and may provide a potentially undetectable, lethal hazard.
U.S. Pat. No. 4,010,324 to Jarvis et al. discloses a background noise masking system intended to overcome some of the difficulties encountered with the Torn system by providing pairs of loudspeaker drivers driven by a noise signal generator having first, second and third time delay blocks where a first set of loudspeaker drivers is driven by the noise generator without delay, a second set of drivers is driven by the noise generator signal having one time delay, the third set of drivers is driven by a signal having two stages of time delay and the fourth set of drivers is driven by a signal having three stages of time delay. Accordingly, the noise, in theory, would tend to be uncorralated from place to place in the open plan office, permitting a more uniform and diffuse sound field, thereby enhancing the psycho-acoustic result as perceived by workers in the office space, since there is relatively low correlation between the noise masking signals coming from any two speakers.
The Jarvis et al. system still has a number of the disadvantages alluded to above, namely, the installer is required to install a number of directional, heavy loudspeaker drivers in the plenum of a suspended ceiling where each of the drivers includes either a paper or plastic cone (and hence is flammable) and each of the drivers must be suspended in some fashion in a ceiling tile or the like.
Others have attempted to overcome difficulties with plenum mounted noise masking loudspeakers by attaching the masking noise system drivers to other parts of the office. In particular, U.S. Pat. No. 4,098,370 to McGregor et al. discloses a system in which a diaphragm speaker and transfer member are directly attached to a structural member such as an office wall, to force vibrations through the structural member. The McGregor et al. system thus requires an installer to determine the frequency response of a wall or other structural member which was never intended to be an acoustic transducer and adjust the masking noise spectrum to provide a uniform sound distribution of pink noise or the like from the structural member. This puts the installer in the position of having to perform acoustic tests on walls, doors and ceilings in an effort to permanently install an effective noise masking system. For a number of reasons, the McGregor system has not found success in the marketplace.
Others have provided masking sound generators which are affixed to * doors or the like. In particular, U.S. Pat. No. 4,052,564 to Propst et al. discloses a masking sound generator resembling a ball which fits upon the top of a door, cubicle partition or the like. The loudspeaker within the noise masking system radiates upwardly or downwardly into a circumferential slot and therefore provides a circular band of noise masking radiation. Since the sound masking generator of the Propst et al. patent must be mounted where it can be easily seen, it would be difficult to make the sound masking system more conspicuous. Others have attempted to overcome problems associated with having a conspicuous external cabinet by incorporating additional and possibly unnecessary features. In particular, U.S. Pat. No. 5,360,469 to Baron et al. discloses an apparatus for enhancing the environmental quality of work spaces combining a high efficiency air filter with a fragrance producing element, blower and sound masking device for generating pink noise, all in a single cabinet, thereby providing a number of different, allegedly work enhancing stimuli. While the Barron et al. apparatus may be pleasant and provide some novelty value in the home, it would hardly be suitable for use in an office, since it requires desk top space and therefore would likely to be used only in and among cubicles and not between them.
There is a need, therefore, for an effective sound masking system which provides a broad-band, diffused sound field but is easily mounted in a plenum.